(1) Field of the Invention
The present invention relates to a heterojunction bipolar transistor and, more particularly, to a transistor having a SiGe (Silicon germanium) base.
(2) Description of the Related Art
With the advancement of self-alignment techniques and fine processing techniques, the speed of operation of Si (Silicon) bipolar transistors has been increasing. For further increasing the operation speed, there are attempts to improve the amplification factor (current gain) and also reduce the base resistance of the heterojunction bipolar transistor (hereinafter referred to as "HBT") by realizing the HBT with a material based on silicon. Particularly, an HBT with the base thereof using a SiGe layer having a narrow band gap is advantageous from the standpoints of the reduction of the power supply voltage owing to operation at low temperatures and also from its utility for low temperature BiCMOSs.
With the conventional bipolar transistor structure, the impurity concentration of the emitter thereof is higher than that of the base. In such a structure, the base-emitter junction has a greater electron barrier than the hole barrier due to the band gap narrowing. When this transistor is cooled down to a lower temperature, the barrier against electrons becomes greater, thus resulting in the reduction of the current gain h.sub.FE and also in the reduction of the cut-off frequency f.sub.T. In an HBT structure having a SiGe base, in order to ensure low temperature operation, the impurity concentration of the base region is made higher than that of the emitter region, while setting these impurity concentrations in a range free from freezing-out, for instance, 3.times.10.sup.18 cm.sup.-3 or above. Under this condition, the barrier against hole in the base-emitter junction is great, while the barrier against electron therein is small. In addition, with lowering the temperature the band gap difference is increased, so that the h.sub.FE is increased and the f.sub.T is no longer reduced. Further, with an HBT with the base thereof constituted by SiGe, which provides a narrower band gap than that of silicon, the barrier against electron in the base-emitter junction is further reduced and, as a result, the h.sub.FE is further increased and the emitter-base junction diffusion potential (V.sub.F) is lower as compared to the base of a silicon homo-junction structure.
The reduction of V.sub.F is an important factor for low temperature operation of the BiCMOS gate circuit. FIG. 1 shows a prior art example of such BiCMOS circuit. Referring to FIG. 1, designated at numerals 12 and 13 are NPN bipolar transistors, at 14 is a P-channel MOS transistor, at 15 to 17 are N-channel MOS transistors, at 18 is an input terminal, and at 19 is an output terminal. In this BiCMOS structure, the high level side voltage loss due to the V.sub.F in the bipolar transistor for pull-up and the low level side voltage loss in the bipolar transistor for pull-down, lead to a corresponding amplitude reduction, thus greatly reducing the operation speed of the BiCMOS gate circuit. In order to reduce the power supply voltage and operation temperature of the BiCMOS gate, therefore, reducing V.sub.F of the bipolar transistor concerned is very important for maintaining a high driving capability to the load and a high operation speed of the BiCMOS.
In order that the emitter-base junction diffusion potential (V.sub.F) be as low as possible in the SiGe base, the content of Ge may be increased. However, if the Ge content is increased excessively, it leads to dislocation in the interface between the silicon substrate and the SiGe due to the difference in the lattice constants between Si and Ge. The lattice constant difference between Si and Ge amounts to 4%. Therefore, the thickness of SiGe that can be grown on Si without dislocation is limited, that is, dislocation takes place when a certain critical thickness is exceeded.
FIG. 4 shows the relation between the critical thickness and the Ge content. As is shown, if a band gap shift of 300 mV, for instance, is to be obtained, a Ge content of 30% is necessary. In this case, the critical thickness of the SiGe is 10 nm. This means that the emitter-collector breakdown voltage is 2.about.3 V, which is below the power supply voltage used. It may be thought to improve the emitter-collector breakdown voltage by increasing the base impurity concentration, for instance, to 5.times.10.sup.19 cm.sup.-3 or above. Doing so, however, reduces the base-collector and base-emitter breakdown voltages.